This invention concerns a method of braking a vehicle driven by an electric motor.
The industrial trucks of today that are used indoors are usually provided with an electric motor in the form of a DC-motor driving a drive wheel via a gearbox. The tuck is further provided with a number of pivot wheels and/or support wheels.
The truck is among other things maneuvered by means of a throttle pedal or accelerator that, dependent on its position, sends signals to an electronic control unit that forwards signals to the control unit of the motor regarding suitable rotational speed for the driving of the truck with the chosen travel speed.
The truck is further maneuvered with a brake pedal that influences hydraulically controlled mechanical brakes that can influence support wheels as well as drive wheels with suitable torque in dependency of the position of the brake pedal.
The truck can also be braked by the driver changing a travel direction switch to opposed travel direction, or alternatively by releasing the accelerator, so called auto-braking, upon which the electronic control unit delivers a signal to the motor control unit to brake the engine with a suitable torque.
These solutions lead to a variation of the braking distance, depending on how heavily loaded the truck is since a heavier load requires greater breaking torque to achieve a given break distance.
Further there is the risk that such a high braking torque is applied that the wheels lock which in addition to leading to longer braking distance also leads to increased wear on the wheels as well as the gearbox. The wheels also risk becoming locked where the friction is insufficient, e.g. at an oil spot, where it thus will be locked when sufficient friction again exists which can give the mentioned damages. Furthermore this can lead to technical control problems since the control system can get the impression of the truck standing still when the wheel no longer rotates and therefore interrupts the braking which then can lead to serious accidents.
Regenerative return of braking energy to the battery can only take place at xe2x80x9cmotor brakingxe2x80x9d using travel direction switch or released accelerator.
The object of the invention is to provide a method to brake an industrial truck where the brake distance is relatively independent of the load of the truck, where maximum brake torque can be limited in order to prevent overloading of the gearbox, and where the risk of wheel locking is minimized.
The object of the invention is achieved by controlling the motor so that its rotational speed is reduced according to a predetermined function of the time the deceleration of the motor is thus controlled and therewith also the deceleration of the driving wheel and the track according to a predetermined characteristic. Accordingly a constant brake distance is obtained independent of load since the control unit controls the motor with the braking torque required to achieve the desired deceleration, where tis the braking torque varies with the load.
By choosing a suitable characteristic a deceleration can be obtained that at permitted loads gives a braking torque that do not cause damage to the gearbox and minimizes the risk of the wheel becoming locked since it has a rotation speed proportional to that of the motor.
Regenerative feedback of brake energy to the battery can take place in some known way.
In one embodiment of the invention different deceleration characteristics can be used dependent on the activated position of the control means. Thus the truck can be triggered to a faster braking (greater deceleration) the more the brake pedal is depressed.
In another embodiment of the invention supplementary brake systems can be triggered when the brake pedal is depressed further than a predetermined position. Hence the motor need only to be fed with the braking current/torque that is required in addition to the braking torque of the supplementary system/systems and a relatively powerful deceleration can be achieved.